These studies are designed to explore the mechanisms involved with growth hormone feedback regulation. The feedback of growth hormone will be studied in normal subjects by infusing various concentration of growth hormone and searching for inhibitory effects on ultradian GH secretion, GRH-induced GH secretion, and insulin hypoglycemia-induced GH secretion. Neurotransmitter modulation of these effects will be evaluated with agonists and antagonists known to influence GH secretion.